As Dee said, you've grown and changed.
In the time you have been apart, you have healed some and no doubt lowered your defenses quite a lot, your anxiety has eased?
Now he reappears next door, as you say, you will probably see him from time to time.
So while it does seem right now to be a big setback in that healing, you may find that you will get back on much more solid ground much sooner than you think, because of the healing you have already done.
It can sometimes help to take another perspective as well. He can't hurt you anymore, not if you don't let him. Sometimes it helps to visualize yourself with a shield around you, maybe like a bubble. He can't talk to you, he can't get through the bubble, you are safe. You are not alone either. You have us and on a more practical level, you have the police. If he ever does decide to come knocking at your door and you tell him (through the door) to go away and he doesn't, immediately (you only have to ask once) just call them. Don't hesitate, they will be happy to help you.
You may also want to put on light blocking curtains on that side of the house, if you have windows there, or that film that shows a mirror look to the outside so no one can see in, but you can see out.
I know it's a shock, I have been through something like that once. Broke up with guy - hadn't spoken for weeks - and I work a side gig a few hours a week, online. For that job there is a group chat so we can keep updated on any changes, ask questions etc. When we were in a relationship, I had referred him, months and months earlier.
Guess who showed up one day in work chat? Yes, the idiot. Plus he has some woman with him as well that he seemed to know. After a couple of days I just stopped logging in to the chat and didn't for months, by the time I returned he was gone.
Anyway I just shut down one more avenue for him to have access to my world, I was fine. As a plus I think I got $200.00 because he signed up on my referral lol
He opens a door, you shut it in his face.